With the development of optical storage solutions and the increasing data capacity of the optical discs, such as VCD (video compact disc), DVD (digital versatile disc), or BD (Blu-ray disc), the speed of recording data on said optical discs has become a key factor.
Some techniques are known for recording data on an optical disc at high speed, in particular techniques using a plurality of laser beams that record data in parallel. Such techniques propose that the plurality of laser beams are controlled so as to simultaneously record data on an optical disc, then simultaneously shift to a next position after said plurality of laser beams has rotated by a pre-defined angle, such as a multiple of 360°.
These known techniques have some major limitations. Indeed, when the plurality of laser beams is shifted by said pre-defined angle, the new data sequence to be recorded by each laser beam is likely to be located inside a data segment (pre-defined on an optical disc). This implies that the laser beams must be controlled with a high precision of less than a channel bit length for linking consecutive data sequences to be recorded. Reaching this high precision is not only difficult to achieve, but also expensive. Moreover, if this high precision is not reached, consecutively recorded data sequences cannot be linked continuously, leading to recording errors because of overlap in data sequence data, and/or to data lost.
However, recording data on an optical disc from a start of a segment requires a lower precision, e.g. just control of a laser pulse of a laser beam at the scale of a wobble wavelength. In this case, it is necessary to detect the start of segments (pre-defined in the spiral of an optical disc) to start recording data on an optical disc, so as to seamlessly link the data sequence recorded by a laser beam and a subsequent data sequence recorded by a next laser beam.
Many conventional, known methods of detecting the start of segments are only based on pre-groove information in spiral tracks of an optical disc. But using only the pre-groove information in the spiral of an optical disc, starts of segments ca not be detected instantaneously, since a laser beam needs to read a minimum length of the pre-groove information, e.g. read at least ⅓ segment (pre-defined in the spiral of an optical disc) to get the position information of the laser beam when dealing, as an example, with the Blu-ray Disc standard. Indeed, with such a standard, a segment (the segment is pre-defined in the spiral of a Blu-ray Disc; after recording, the segment will be a RUB (recording unit block)) is equivalent in length to 3 ADIP (Address In Pre-groove) words. If less than a full length ADIP word is scanned (i.e. less than ⅓ of a segment), the start of a current segment cannot be detected, so the recording can only be started from a next start of segment. That is to say, the laser beam has to wait a longer time to start recording.
FIG. 1 is a schematic diagram illustrating three continuous segments (S1, S2, S3) on an optical disc, each segment comprising 3 ADIP words (ADIP1, ADIP2, ADIP3).
It shows a laser beam B located inside the word ADIP3 of segment S1, meaning that the distance between the current position of laser beam B and the Start_S2 of the following segment S2 is less than the length of one ADIP word. As a consequence, the position on the optical disc of segment S1 cannot be detected, so that the recording cannot be started at the start of the following segment S2. The laser is thus obliged to scan at least one full ADIP word of the following segment S2 for determining the position of segment S2 on the optical disc. The data recording by laser beam B can thus only be started at the Start_S3 of segment S3, meaning that the laser beam B is inactive throughout segment S2. The process of recording is thus not optimal.